This invention relates to electrical safety circuits designed to protect personnel against injury by shock in the event of accidental contact with current-carrying conductors. More particularly, the circuitry relates to a type of protective circuit now generally referred to as the "Bodkin Circuit", (U.S. Pat. No. 3,997,818), which affords special as well as conventional protection, detecting as fault current an accidentally induced or unintended current flow between the hot and the return or neutral conductors in a power cable, thereby providing a substantially universal protection wnich includes, but is not limited to, detection and reaction to fault current flow from the hot wire to a ground other than that represented by the return conductor, as conventional ground fault circuit interrupters are limited.
While this added feature of "line-to-line" or "load fault" protection in the transmission of electric power provides additional shock protection to a person by protecting in circumstances beyond the control capabilities of conventional or prior state-of-the-art equipment, it also in a similar fashion serves to protect from a common cause of household fire in which deteriorating insulation can result in line-to-line arcing where insulation between conductors may be deteriorated by physical abuse and also by heat, which is often generated at resistance areas which may result from physical abuse. This type of protective circuit is also inherently less sensitive to false tripping caused by certain types of line and load-originating transient conditions, and is capable of being operated reliably over a broader range of sensitivity, adjustable to permit substantial leakages in circumstances where such may be desirably and safely tolerated before turn-off, but capable of being made to react to fault currents of less than one milliampere where even minute leakage faults may be considered undesirable, as in some circumstances involving medical equipment or specialized industrial testing or processing devices. It also is able to monitor fault presence after power to the load has been interrupted, and to automatically restore power once the fault has been corrected or cleared.
In the prior circuits disclosed in U.S. Pat. No. 3,997,818, preferred load-current protective switching has been accomplished by means of directly and indirectly controlled solid-state devices or thyristors such as silicon controlled rectifiers. While economica- to employ where current demands are not great, such devices capable of carrying heavier currents usually above 35 amperes, appear to rise almost exponentially in cost with further increases in capacity. Even with improved performance and tolerances to abuses, including the ability to withstand momentary surge currents that far exceed their rated capacities, they still suffer criticism from many areas in regard to their susceptibilities to surge-related damage and also their tendency to distort wave form and/or introduce harmonics. However, the "full-on" typical operating characteristic of this circuit principle tends to avoid the problems associated with phase control. Derating is a means to increase resistance to abuse which has not been economically feasible in the case of the switchable solid state devices such as the SCR due to the aforementioned rapid rise in cost associated with higher ratings. Where heavy load currents must be carried by solid state devices in accordance with the present invention, they are not switched by such devices, they are only rectified by the simpler diode devices which are more readily and economically available in very high ratings and which therefore can be selected to have excessive capacities for higher tolerance to such abuses at acceptable cost. The invention includes means to initiate a turn-off of the supplied power of such a rectifier fails in its ability to rectify, but remains conductive. While solid state switching devices are employed in the control circuitry of this invention, they are relieved of high current carrying requirements and are substantially isolated from the load and line transients which would comprise abuse and may thus be also readily provided in a convenient and inexpensive integrated circuit form. Where heavy currents are to be switched, the solid state semi-conductor circuits control more conventional electromechanical relay means which are utilized in a normally open contact armature arrangement, as shown in the preferred embodiments, so that relay failure would be anticipated to be in the open state.
When the conductors in a power cable are cut, free and contactable ends can be exposed including those of that portion of the cable still connected to the power source and of that portion still connected to the device comprising the load. Protection provided by the prior art, as in U.S. Pat. No. 3,997,818, includes protection in the event of contact with such ends when placing the body in a connective position between said conductors or even between these conductor ends and the exposed ends of the conductors still attached to the load, but special situations may occur in which the cable is only partially cut. One such situation is when the hot wires remain intact and connectively coupled to the load and the ground becomes interrupted so that the portion connected to the load and thus to the power source through the load becomes exposed and contactable. Simultaneous contact with this exposed end and the ground can permit current to flow through the body from the power source in series with the load and its identifying diode or diode bridge and not cause a by-pass of tne diode in the load to provide reverse current and initiate turn-off as it would if one of the hot leads were contacted. This represents a possible shock circumstance for which protection in the aforementioned patent was obtained by passing an initial warning current and substantially delaying turn-on time of load current but is a circumstance for which protection, in accordance with the present invention is provided without requirement for such delay and the delaying devices.
While conventional GFCI protected circuitry can trigger turn-off when the current from a severed ground side conductor still connected to the load is diverted to a ground other than that represented by the return conductor, the improved circuits of this invention detect and react to fault current flow even when undiverted or when the ground of a reconnection after severance is provided by the return conductor, for which protection is conventionally or otherwise unavailable.